Process for controlling the molecular weight of polymers of bromostyrenes

ABSTRACT

A controlled molecular weight polymer of styrene is provided having bromine substituted thereon. Control of molecular weight is achieved by the use of alpha-methyl styrene dimer as a chain transfer agent. The brominated polymer of styrene is useful as a flame retardant, particularly for polyamides giving improved properties including color retention after molding.

CLAIMING BENEFIT

This application claims priority from U.S. Provisional Application No.60/615,132, filed Sep. 30, 2004, entitled “CONTROLLED MOLECULAR WEIGHTBROMOSTYRENES.”

FIELD OF THE INVENTION

Novel homopolymers and copolymers of bromostyrene monomers are preparedusing α-methyl styrene dimer (MSD) as a chain transfer agent to controlmolecular weight. The high temperature polyamide (HTPA) productscontaining these materials provide superior color retention aftermolding under high temperature conditions compared with materials thatwere prepared without the MSD chain transfer agent.

BACKGROUND OF THE INVENTION

Bromostyrene polymers are known flame retardants for a variety ofplastics, but have especially found use in the polyamide family due totheir remarkable thermal stability. Commercially available homopolymersand functionalized copolymers of mixed mono-, di-, and tribromostyrenes,available from Great Lakes Chemical Corporation, are the products ofchoice for HTPA. The use of bromostyrene homopolymers in HTPA isdescribed in numerous patents and publications, while functionalizedbromostyrene-glycidyl(meth)acrylate copolymers in HTPA are disclosed inWO 02/24812 to Martens et al. of Dupont and in U.S. Pat. No. 6,414,064B1 to Matsuoka and Sasaki of Kuraray.

HTPA incorporating bromostyrene polymers suffer from compromised meltflow. Improved molecular weight control for the bromostyrene polymerscan provide increase the melt flow of the flame retarded HTPAcomposition, thereby improving the processability without compromise offlame retardancy. As taught in U.S. Pat. No. 5,304,618 to Atwell et al.of Great Lakes Chemical Corp., the molecular weight of thepolybromostyrenes has typically been adjusted using 1-dodecanthiol as achain transfer agent added during polymerization. For a period of time ahomopolymer of mixed mono-, di- and tribromostyrenes having about 60 wt% bromine and a weight average molecular weight (Mw) of about 8,000 wascommercially available from Great Lakes Chemical Corp. It was cataloguedas “PDBS-10” and was marketed to the polyester and polyamide area. Thislow molecular weight/high melt flow polymer was produced by the use of athiol chain transfer agent.

The use of α-methyl styrene dimer as a molecular weight regulator forfree radical stryrenic polymerizations is known. See, for example, U.S.Pat. No. 5,559,200 to Suzuki et al. of Hitachi Chemical. Further, at apoint many years after commercial introduction of PDBS-10 describedabove, JP 08-188622 to Horie and Kagawa of Tosoh Corp. restated thatpolymers of bromostyrene with an MW of between 1,000 to 10,000 haveimproved compatibility, flow, thermal discoloration resistance andelectrical properties. The application notes that the benefits can berealized by solution polymerizing bromostyrene in the presence of achain transfer agent to obtain an Mw of less than 10,000. While thispublication does disclose that MSD may be used as the chain transferagent, it does not indicate that any particular advantage is obtained inusing MSD over the alkyl mercaptans (thiols) or alkyl halides that aredescribed. Further, it does not teach the desirability in using MSD toproduce polymers of bromostyrenes having an Mw of greater than 10,000.

While the known bromostyrene polymers serve as flame retardants for hightemperature polymers, a need still exists for a polymeric flameretardant which improves processability high temperature polymers,particularly HTPA. An advantage found from the bromostyrenes disclosedis a reduced discoloration of the composition in the event it isprocessed at an elevated temperature for an extended period of time.

SUMMARY OF THE INVENTION

It is an object of the invention to provide polymers of ar-bromostyrenemonomers having the following general structure:

where R is H or CH₃ and x is an integer of 1 to 5. Preferably, onaverage, X is 2 or more. As X approaches 5, the time for bromination isextended. Average values of X of 2.5 to 3.5 are found useful. The prefix‘ar-’ used herein means substituted on an aromatic ring. Further, thesepolymers have an assigned weight average molecular weight (Mw) comparedwith a polystyrene standard as measured by gel permeation chromatographyof from about 11,000 to about 60,000. Moreover, the polymers may furthercomprise residues of α-methyl styrene dimer.

It is another object of the invention to provide bromostyrene polymersby polymerization of styrene in the presence of MSD, followed bybromination.

It is another object of the invention to provide an essentiallysolvent-free process to control the molecular weight of bromostyrenepolymers in which α-methyl styrene dimer is used as a chain transferagent.

It is yet another object to provide improved high temperature flameretardant polyamide compositions comprising bromostyrene polymers whichcontain residues of α-methyl styrene dimer.

DETAILED DESCRIPTION OF THE INVENTION

Polymers of the present invention comprise units of ar-bromostyrenes orα-methyl bromostyrenes having the structure I where R is H or CH₃ and xis an integer of 1 to 5. Preferably the units are ar-bromostyrenescontaining 2-4 bromine atoms per molecule, and most preferably thepolymers are formed from mixtures of ar-bromostyrenes having an averageof about 3 bromine atoms per molecule.

While it is most preferred that the polymers be derived from thepolymerization of aromatically brominated monomers, they can also bemade by the post-bromination of lower molecular weight polystyreneprepared using α-methyl styrene dimer as a chain transfer agent. Theprocess of bromination of simple polystyrene is well-known in the flameretardant industry.

The polymers of the present invention may contain a lesser number ofunits other than the ar-bromostyrenes. These may be any other moleculecapable of copolymerizing with the ar-bromostyrene and could beadvantageously included to modify the compatibility of the flameretardant polymer with a particular base resin requiring reducedflammability. Examples of possible functional co-monomers include, butare not limited to, glycidyl(meth)acrylate, maleic anhydride,hydroxyethylmethacrylate, acrylic acid, ar-amino substituted styrene,and styrene sulfonic acid and its salts.

If the addition of a comonomer is desired to enhance compatibility ofbromostyrene with the polymer matrix to be flame retarded, then it ispreferred to provide sufficient amount of comonomer to provide thedesired improvement in compatibility, but not so much as tosignificantly dilute the amount of bromine in the composition. Thecomonomer is believed to provide improved compatibility when comprisingat least about 0.1 10 mol % but no more than 10% mol based on the amountof bromostyrene used, co-monomer amounts at least about 0.5 mol % but nomore than about 5 mol % being more preferred.

Besides the inclusion of functional co-monomers, it is also possible tocopolymerize the bromostyrene with other monomers in order to modify thecharacter of the final product. Some examples of potential nonfunctionalco-monomers include styrene, butadiene, acrylonitrile,methyl(meth)acrylate, divinylbenzene, isoprene, n-butylacrylate,α-methyl styrene, and p-methylstyrene.

The resulting copolymer, depending more so on the selection ofbromostyrene used, should preferably contain at least about 50 wt %bromine but no more than about 80 wt % bromine, more preferably at leastabout 55 wt % but no more than 75 wt % bromine, and most preferably atleast about 60 wt % bromine but no more than 70 wt % bromine.

The bromostyrene polymer is preferably of a moderate to low molecularweight in order to impart improved flow properties to the totalcomposition. For the purposes of this disclosure, molecular weight isdefined in terms of weight average molecular weight (Mw) compared with apolystyrene standard as measured by gel permeation chromatography (GPC).This means that for a given sample of poly(bromostyrene), orbromopolystyrene, it is assigned the Mw of a known polystyrene standardhaving the identical GPC retention time. This is believed to indicatethat the average chain length of the poly(bromostyrene) and thepolystyrene standard are about the same, but that this does not takeinto account the mass contributed by the bromine atoms, so truemolecular weight would require further calculation based on the averagenumber of bromine atoms per ring. The assigned Mw of the bromostyrenepolymers of this invention will preferably be at least about 11,000 butno more than about 80,000. More preferably they will range from about13,000 to about 60,000. Most preferably the bromostyrene polymer willhave an assigned MW of from about 15,000 to about 20,000.

The invention makes advantageous use of an α-methyl styrene dimer tocontrol the molecular weight of the bromostyrene polymer during freeradical polymerization. Applicant found advantageous propertiesresulting in the use of the α-methyl styrene dimer that were notavailable from the prior art mercaptan or aliphatic halide chaintransfer agents. The α-methyl styrene resulted in an improved stabilityduring molding. Although not wishing to be bound by any theory, itappears that use of chain-transfer agents containing heteroatoms(sulfur, halogen, oxygen, for example) tend to lessen the thermalstability of the resulting polymer system. Other hydrocarbon materialssuch as substituted MSD types of products, toluene, and others known toone skilled in the art may be used. MSD is preferred due to its higherefficiency relative to the others.

Commercially available α-methyl styrene dimer may not be a single, purematerial. U.S. Pat. No. 6,388,153 B2 to Gridnev of Dupont teaches thatthere are at least two major isomers that may be present. See structuresII and III. According to the U.S. Pat. No. 6,388,153 the isomer instructure III with the “external” double bond is the one which functionsbest as a molecular weight modifier. Although mixtures of the isomersmay provide the objectives of this invention, it is preferred that dimerwith the highest possible concentration of structure III isomer be used.

Isomers of Alpha-Methyl Styrene Dimer

The U.S. Pat. No. 6,388,153 also teaches that the aromatic rings may besubstituted with various reactive functional groups. This may be highlydesirable when used as a chain transfer agent in bromostyrenepolymerizations. Poly(bromostyrene)homopolymer has poor compatibilitywith most plastics.

Scanning electron micrographs of molded HTPA show what appears to bediscreet spheres or globules of bromostyrene in the matrix of HTPArather than a homogeneous blend. This separation apparent on electronmicrographs of cooled moldings appears in spite of apparent completedispersion of bromostyrene in the HTPA melt. The addition of reactive orcompatibilizing polar groups to the bromostyrene polymer via theintroduction of functional comonomers previously described has beentaught to reduce the two-phase appearance in micrographs as compared tohomopolymer.

As taught by Watanabe et al. (Addition-Fragmentation Chain Transfer inFree Radical Styrene Polymerization in the Presence of2,4-diphenyl-4-methyl-1-pentene, Chemistry Letters (Japan), 1992, pp.1089-1092) residues of the α-methyl styrene dimer chain transfer agentdo become part of the polymer composition. Therefore, it is believedthat when using a selectively derivatized α-methyl styrene dimer, thedimer residues—which may be present at a terminal of many of thebromostyrene chains—could carry a reactive or polar compatibilizinggroup attached to the ring which will improve the compatibility of thebromostyrene polymer with the matrix polymer requiring flame retardancy.For example, the U.S. Pat. No. 6,388,153 says that both rings of theα-methyl styrene dimer may contain one or more —NH₂ or —N═C═O groups.Either of these groups can be expected to improve compatibility withpolar matrix polymers such as polyesters and polyamides. It can also beexpected that if the rings carry other polar groups compatible withpolymers having a polar nature, that compatibility will be improved.Examples of such polar substituents which might be bound to the MSDaromatic rings include amines, carboxylic acid and its salts, amides,esters, and epoxides.

The bromostyrene polymers of the present invention may be prepared usingfree radical methods known in the industry. Polymerizations may beconducted in batch, semi-continuous, or continuous fashion. Thereactions may be run in the presence of a solvent, or in an essentiallysolventless system as described in the U.S. Pat. No. 5,304,618 mentionedabove. For the purposes of speed and economy, the solventless process ispreferred. Any of the common free radical initiators such as peroxy andazo compounds may be used to accelerate the reaction and to reduceresidual monomer, but thermal initiation and polymerization is possible.(A review of free radical initiators can be found in The Encyclopedia ofPolymer Science and Technology, 3^(rd) Edition, Volume 6, pp. 563-600.)For the sake of economy and efficiency, the reaction is preferably runin a continuous fashion, without the use of solvent, and in the presenceof a free radical initiator; such a method is taught in U.S. Pat. No.5,304,618, incorporated herein by reference.

The bromostyrene polymers with controlled molecular weight are useful asflame retardants for any thermoplastic polymer, but are especiallysuited for use in polyester or polyamide resins. The most preferredpolyesters are polyethylene terephthalate and polybutyleneterephthalate. Any polyamide may be used as a matrix resin, includingpolyamide-6 and polyamide-66, but those polyamides having melttemperatures above about 280° C. will especially benefit from thestability of the new bromostyrene polymers. Such high temperaturepolyamides are described in U.S. Pat. No. 5,115,010 to Sakai et al. ofMitsui Petrochemical Industries. Examples of such polymer compositionsinclude polyamide-4,6, polyamide-4,8, polyamide-4,9, polyamide-4,10,polyamide-4,11 polyamide-4,12, polyamide-4,13, polyamide-4,14 and thesemi-aromatic polyamides such as polyamide-6,6/6,T,polyamide-4,6/4,T/4,1, polyamide-9,T, polyamide-12,T.

Other additives may be present in the final polymer composition. Thesemay include fillers, reinforcing agents such as glass fiber, colorants,stabilizers such as hydrotalcite, flow enhancers and flame retardantaids such as antimony compounds and zinc borate.

EXAMPLES Example 1 Compatibilized Copolymer Using Thiol

To simplify preparation of relatively small amounts of polymer, alaboratory scale process based on a solution polymerization method wasused. A 2-liter 4 neck flask was equipped with mechanical stirring,heating mantle, thermocouple probe, and a condenser. The flask was thencharged with mixed brominated styrene monomer containing about 64%bromine (995.2 g), dichlorobenzene (487.3 g), glycidyl methacrylate(5.08 g, 0.0357 mol), dodecanthiol (7.85 g, 0.0387 mol) as a chaintransfer agent (CTA) and 2,2′-azobis(2,4-dimethylvaleronitrile) soldunder the tradename VAZO-52™ azo type initiator (1.00 g, 4.03 mmol)available from DuPont. The solution was then heated to about 90° C.until it exothermed. Peak temperature reached was 146° C. after whichthe mixture was heated to 190° C. over 5-10 min and held at 190° C. for40-45 min. Heating was halted; the solution (1484.4 g) was transferredto a 2-liter bottle, and cooled to room temperature. The material wassplit into four parts. Each part was precipitated into a 5-liter flaskof 3,400 mL acetone and 400 mL methanol. The precipitate was filteredover frit, dried at ambient temperature for about 14 h and oven dried at110° C. for 8 h. A white powder (842 g) was isolated.

Example 2 Compatibilized Copolymer Using α-Methyl Styrene Dimer (MSD)

Using the same procedure and equipment as in Example 1, the flask wascharged with brominated styrene monomer (1004.2), dichlorobenzene (492g), glycidyl methacrylate (5.11 g, 0.0359 mol),2,4-diphenyl-4-methyl-1-pentene (20.13 g, 0.0851 mol) (α-methyl styrenedimer) and VAZO-52™ (1.03 g, 4.15 mmol). The solution was heated toabout. 90° C. until exotherm. It then reached a peak temperature of 141°C. after which the mixture was heated to 190° C. over 5-10 min and heldat 190° C. for 40-45 min. Heating was halted; the solution (1509.3 g)was transferred to a 2-liter bottle, and cooled to room temperature. Thematerial was split into four parts. Each part was precipitated into a5-liter flask of 3400 mL acetone and 400 mL methanol. The precipitatewas filtered over frit, dried at ambient temperature for about 14 h andoven dried at 110° C. for 8 h. A white powder (881 g) was isolated.

Example 3 Compatibilized Copolymer Using α-Methyl Styrene Dimer (MSD)and Initiator Blend

A high temperature initiator, cumene hydroperoxide, was included in thisrun to drive the polymerization further towards completion and to reduceresidual monomer. The flask was charged with brominated styrene monomer(1002.6), dichlorobenzene (497 g), glycidyl methacrylate (5.13 g, 0.0360mol), 2,4-diphenyl-4-methyl-1-pentene (20.26 g, 0.08571 mol), VAZO-52™azo type initiator (1.05 g, 4.22 mmol) and cumene hydroperoxide (1.06 g,6.96 mmol). The solution was then heated to about 90° C. until exotherm.Peak temperature reached was 163° C. after which the mixture was heatedto 190° C. over 5-10 min and held at 190° C. for 40-45 min. Heating washalted; the solution (1513.7 g) was transferred to a 2-liter bottle, andcooled to room temperature. The material was split into four parts. Eachpart was precipitated into a 5-liter flask of 3400 mL acetone and 400 mLmethanol. The precipitate was filtered over frit, dried at ambienttemperature for about 14 h and oven dried at 110° C. for 8 h. A whitepowder (883 g) was isolated.

Example 4

Styrene (1012.39), chlorobenzene (1483.9 g), 1-dodecanthiol (12.0 g),and Vazo® 52 (1.092 g) were charged to a 3 liter 4 neck flask equippedwith a mechanical stirrer, THERM-O-WATCH®, thermocouple, heating mantleand condenser. The mixture was stirred for 14 minutes at roomtemperature and then heat was applied. The temperature was raised to 80°initially during which time the polymerization initiated and exothermedto 85° C. The temperature was then raised to 130° C. and held for 10hours. Heat was removed and stirring continued until the temperature wasat 90° C.

The reaction mixture was precipitated into methanol (1 mL of reactionmixture per 12 mL of methanol). The product was filtered, dried at 75°C. for 24 hours, cooled and bottled. The yield was 497.4 g ofpolystyrene.

Under a slow nitrogen purge, polystyrene (477 g) and ethylene chloride(2 L) were charged to a dry 5 liter 4 neck flask equipped with amechanical stirrer, thermocouple, and condenser. The reaction flask wasvented to a scrubber charged with 10% sodium sulfite. The polystyreneslurry was stirred at ambient temperature until a homogeneous solutionwas obtained. The reaction mixture was then chilled in an ice-bath.Aluminum chloride (9.2 g) was charged to the reaction mixture in oneportion. Bromine (717.8 g) was slowly added over 3 h by pump, whilemaintaining a reaction temperature of 15-20° C. The resulting mixturewas stirred overnight at ambient temperature. The following day themixture was quenched with 1 L water and 50 mL of 50% sodium hydroxide.Temperature was controlled by use of an ice-bath. The product wasisolated from the organic phase by azeotropic distillation of ethylenechloride from boiling water (7 L). The product was filtered, washed withwater, and dried.

Example 5

A repeat of Example 4.

Example 6

Styrene (1000.0 g), chlorobenzene (1581.4 g), α-methylstyrene dimer(20.0 g), and Vazo® 52 (1.04 g) were charged to a 3 liter 4 neck flaskequipped with a mechanical stirrer, thermocouple, heating mantle andcondenser. The mixture was stirred for 5 minutes at room temperature andthen heat was applied. The temperature was raised to 80° initiallyduring which time the polymerization initiated and exothermed to 85° C.The temperature was then raised to 115° C. and held for 23 hours. Heatwas removed and stirring continued until the temperature was at 60° C.

The reaction mixture was precipitated into methanol (1 mL of reactionmixture per 14 mL of methanol). The product was filtered, dried at 75°C. for 24 hours, cooled and bottled. The yield was 596.3 g ofpolystyrene.

The polystyrene was brominated as in Example 4.

Properties of the polymers prepared are found in Table 1.

TABLE 1 Properties of Bromostyrene Polymers CTA Mols. Assigned ResidualUsed CTA MW Bromine, % Monomer, % Example 1 Thiol 0.0387 13,400 63.20.58 Example 2 MSD 0.0857 16,400 63.1 0.52 Example 3 MSD 0.0857 14,90063.4 0.21 Example 4 Thiol 0.0592 58400 39.6 N/A Example 5 Thiol 0.059288000 43.3 N/A Example 6 MSD 0.0846 83700 43.5 N/AThese results show that a higher loading of MSD than thiol is requiredto obtain equivalent molecular weight, but not so much more as tosignificantly reduce the overall bromine content of the polymer. Also,the use of an initiator blend seems to have the desired effect ofreducing the amount of unreacted monomer in the final product.

Examples 7 & 8 Thermal Stability of the Bromostyrene Copolymers

Small samples of copolymer weighing about 10 mg from Example 1 (Example7) and Example 2 (Example 8) were subjected to isothermal hightemperature exposure in a TGA Q 500 Thermogravimetric Analyzer from TAInstruments for 30 minutes at 330° C. under nitrogen while their weightwas continually recorded. The stability was determined by measuring themass loss during the first 20 minutes and calculating the % mass lossper minute (Table 2).

TABLE 2 % Mass Loss Rate During 330° C. Isothermal TGA Exposure % Mass %Mass % mass loss/ Retained, Retained, Sample minute 20 minutes 30minutes Example 1. 0.70 84.3 81.2 Example 2. 0.49 88.4 85.9

These results show a clear improvement in stability as measured byisothermal TGA analysis (decrease in rate of mass loss) for theMSD-modified copolymer.

Examples 9 & 10 Thermal Stability of Polyamide Compositions

Bromostyrene copolymers prepared using as chain transfer agent1-dodecanthiol and separately MSD were compounded into a glassreinforced high temperature semi-aromatic polyamide (Examples 9 & 10,respectfully). The compounded resin compositions were then injectionmolded into plaques using two conditions: The first was at a normal melttemperature of about 310° C., while the second was run at about 340° C.to simulate more abusive conditions which might be seen at a typicalmolding operation optimized to push the limits of processingtemperatures in order to reduce melt viscosity and increaseproductivity. Increase time and temperature exposure may occur if amolding operation is temporarily halted, during which time the next shotof compound is being held in a molten state at a relatively hightemperature.

Table 3 shows the normal molding conditions versus the abusiveconditions used to evaluate the thermal stability of the formulationsdescribed hereafter.

TABLE 3 Normal versus Abusive Molding Conditions Normal Abusive RearTemperature, ° C. 304 321 Center Temperature, ° C. 304 321 FrontTemperature, ° C. 310 325 Nozzle Temperature, ° C. 321 338 MoldTemperature, ° C. 88 88

Examples 11, 12, 13 & 14 Compatibilized Copolymers Using MSD with 60 and64% Bromine Content Monomer

Using the procedures described in Ex. 1 and Ex. 2, twobromostyrene/glycidyl methacrylate copolymers were prepared (Table 4)(Examples 11 & 12, respectfully). Bromostyrene having a lower content ofthe tribrominated species was used. Additionally, to investigate theeffects of conducting the polymerization reaction without solvent andunder higher temperature conditions (as described in U.S. Pat. No.5,304,618), some additional copolymers were prepared using 0.5% GMA asshown in Table 4 (Examples 13 & 14, respectfully).

TABLE 4 Additional Bromostyrene/GMA Copolymers Ex. 11 Ex. 12 Ex. 13 Ex.14 Solvent Process Neat Process Chain 1-dodecanethiol MSD1-dodecanethiol MSD Transfer Agent Assigned 18,400 22,800 17,100 19,700MW Bromine 59.0 59.1 64.8 64.0 Content, %

Polyamide formulations of Tables 5 and 5a were processed through aBerstorff ZE 25 twin screw extruder. The glass fiber was side-fed intothe fifth barrel of the extruder using a Brabender loss in weight feederand a K-tron side feeder to reduce fiber breakage. Barrel temperatureswere ramped from 310° C. at the feed throat to 330° C. at the die. Thenominal feed rate was about 35 lbs/hr. The extruded strand was cooled ina water bath and chopped into pellets.

TABLE 5 Polyamide Formulations Made From Polymerized BromostyrenesExample No.: 15 16 17 18 19 20 Polyamide¹, % 45 45 41 41 45 45 GlassFiber², % 30 30 30 30 30 30 Ex. 1, % 20.3 — — — — — Ex. 2, % — 20.3 — —— — Ex. 11, % — — 22 — — — Ex. 12, % — — — 22 — — Ex. 13, % — — — — 20.3— Ex. 14, % — — — — — 20.3 Antimony Trioxide³, % 4.5 4.5 7 7 4.5 4.5Magnesium 0.35 0.35 0.35 0.35 0.35 0.35 Stearate⁴, % ¹Zytel ® HTN fromDupont ²Chopped Strand #3540 from PPG Industries ³TMS-HP ® from GreatLakes Chemical Corp. ⁴Lubricant from Synpro

TABLE 5a Polyamide compositions made from bromination of polystyreneExample No.: 21 22 23 Polyamide¹, % 39.75 39.75 39.75 Glass Fiber², % 3030 30 Ex. 4, % 22.5 — — Ex. 5, % — 22.5 — Ex. 6, % — — 22.5 ZincBorate³, % 7 7 7 PTFE⁴, % 0.4 0.4 0.4 Polyethylene wax⁵, % 0.35 0.350.35 ¹Zytel ® HTN from DuPont ²Chopped Strand #3540 from PPG Industries³FireBrake ® from US Borax ⁴Teflon ® 6C from DuPont ⁵Luwax ® OP fromBASF

Using a Van Dorn 35 ton injection molding machine, the pellets weremolded into plaques measuring approximately 2×3 inches. The normal andabusive procedures described in Table 3 were used.

To quantify the degree of color change in going from normal processingtemperature to the higher processing temperature, readings were taken onboth sets of plaques using a Colorquest Tristimulus colorimeter. L, a,and b values were measured. (For a detailed discussion of colormeasurement see The Encyclopedia of Chemical Technology, 4^(th) Edition,Volume 6, pp. 841-876.) A calculation was applied to the L, a, and bvalues in going from low to high temperature processing to obtain acolor difference reading, ΔE where

ΔE=[(ΔL)²+(Δa)²+(Δb)²)]^(1/2)

The results are shown in Table 6.

TABLE 6 Color Comparison of Formulations Compounded Material ColorDifference Example Polymer, Polymer (Abusive − Normal Number % BrProcess Molding), ΔE 15 63.2 solvent, thiol 10.30 16 63.1 solvent, MSD2.10 17 59.0 solvent, thiol 6.91 18 59.1 solvent, MSD 1.89 19 64.8 neat,thiol 7.00 20 64.0 neat, MSD 0.50 21 39.6 solvent, thiol 3.59 22 43.3solvent, thiol 2.73 23 43.5 solvent, MSD 1.65

The polyamide compositions that contain the thiol-modified bromostyrenecopolymers are a pale yellow/gray color at moderate moldingtemperatures, but when processed hotter yields a markedly gray color.The MSD-modified bromostyrene system is pale yellow at the lowertemperature and the color is maintained at the higher processingtemperature. The color differences for the plaques that contain a ΔEvalue of near 2 or below are difficult to detect with the naked eye.

For polyamide formulations containing the thiol-modified copolymers, the□E values are quite high at the 6.9-10.3 range, whereas the polyamideformulations containing the MSD-modified copolymers, the □E is in the0.5-2.1 range, confirming that there was much less overall color changewhen the copolymers of this invention were used.

1.-17. (canceled)
 18. A process to control the molecular weight ofbromostyrene polymers in which α-methyl styrene dimer is used as a chaintransfer agent, comprising the steps of: (a) providing an essentiallysolventless blend of: (i) monomers of brominated styrenes; (ii) apolymerization initiator; and (iii) α-methyl styrene dimer, (b) feedingthe monomer/polymerization initiator blend into a reaction vessel; (c)reacting the monomer/polymerization initiator blend under conditionseffective to polymerize at least about 80% of the monomers in a time ofbetween about 1 minute and about 20 minutes; and (d) removing thepolymerized bromostyrene from the reaction vessel.
 19. (canceled) 20.The process of claim 18 wherein the brominated styrene monomer is anar-bromostyrene monomer having the following general structure

where R is H and x is 3, where the polymers have an assigned weightaverage molecular weight (MW) compared with a polystyrene standard asmeasured by gel permeation chromatography of from about 11,000 to about60,000.
 21. The process of claim 18 wherein the bromostyrene polymersretain at least 88% of their mass after 20 minutes and at least 85% oftheir mass after 30 minutes when a 10 mg specimen is held at 330° C.under a nitrogen atmosphere.
 22. The process of claim 21 wherein thepolymers have an assigned weight average molecular weight compared witha polystyrene standard as measured by gel permeation chromatography offrom about 15,000 to about 20,000.